In recent years, liquid crystal display devices have been widely adopted as display devices of a computer, a mobile phone, a PDA and a game machine due to the characteristics such as low power consumption, thinness and light weight. Generally, liquid crystal display devices include a liquid crystal display panel and a backlight for illuminating the liquid crystal display panel from the backside. The liquid crystal display panel is formed by sandwiching a liquid crystal layer between an active matrix substrate and a counter substrate.
The active matrix substrate has a configuration in which a plurality of pixels are formed on a glass substrate in a matrix. A TFT and a pixel electrode form a pixel. The counter substrate has a configuration in which counter electrodes and color filters corresponding to respective pixels are formed on a glass substrate. Due to such a configuration, in a liquid crystal display device, a voltage applied to a space between each pixel electrode and each counter electrode is adjusted, thereby the transmittance of the liquid crystal layer is adjusted on a pixel basis. As a result, an image is displayed on a display screen due to illumination light from the backlight that passes through the liquid crystal layer.
As described above, conventional liquid crystal display devices have a function of displaying an image. However, in recent years, display devices also having a function of capturing an image have been proposed (e.g., see Patent document 1). In a liquid crystal display device disclosed in Patent document 1, a plurality of photodiodes are formed on an active matrix substrate in a matrix, and the liquid crystal display panel functions as an area sensor.
Further, in Patent document 1, a PIN diode having a lateral structure is used as each of the photodiodes. With the utilization of a TFT process, each of the PIN diodes is formed by forming a p-layer, an i-layer, and an n-layer in that order on a silicon film common to a TFT.
Structurally, these PIN diodes respond not only to light incident from the observer side but also to illumination light from the backlight. Thus, the PIN diodes are generally provided with a light shielding film on the backlight side so as to block the illumination light from the backlight from entering the PIN diodes. Due to these light shielding films, each of the PIN diodes only responds to light entered the liquid crystal display panel from the observer side, and outputs a signal.    Patent document 1: JP 2006-3857 A (pages 11 to 12 and 20 to 21, and FIGS. 20 and 38)